The heat exchanger just cracked on my 15 year old Goodman. I would like to purchase a new furnace and complete most of the install myself; I have an HVAC friend from work that is willing to help.

Here is the info from the old -

Goodman GMPN128-5REVA L.P

120,000 btu input

21w 18.5d Pletium (top part)

46.5 old furnace

Airdrop

25w 16h

I have been looking around online and see a host of options. We live in Central Iowa and it can get real cold, so I need a reliable, efficient model. I am not brand loyal but I did just install a Rheem tankless water heater and I've been happy with that.

I would like a 96%+ efficentcy rating and would obviously convert a Natural Gas unit to propane for my application. The unit is mounted in my basement and I would like to keep as much of the original ducting as possible and just swap out the furnace unit alone; if possible.

What are your thoughts? Please suggest specifc models so I have no question what your suggesting. Any help is greatly appreciated. Thank you in advance.

You can do the research as easily as anyone else. Your primary constraint is fitting the new furnace into the existing ducting. So I'd start with finding a furnace that will fit physically. Then look to see if there's a natural gas to LP conversion kit. Then look at the efficiency. 90% and above are condensing furnaces meaning they get their last bit of energy from condensing the water vapor in the flue gasses. That will require a system of draining the condensation. It will also mean a more complicated furnace to maintain.

Once you find the right fit then look for dealers. Some online places are more expensive than retail HVAC shops so look carefully. Possibly you can get a furnace through a HVAC supply house like Johnstone or McCalls. Since there no AC involved the refrigerant EPA licensing isn't involved.

Depending on the size and location of the crack in your old furnace, it may be easier to repair it or investigate a replacement heat exchanger.